FISHERY BULLETIN: VOL. 81, NO. 4 



was significantly lower in Thalassia; and lowest ef- 

 ficiency of predation occurred in Syringodium. 

 Therefore, low biomass and high predatory success 

 in the Halodule bed would make it the habitat of 

 choice for pinfish and probably other consumers of 

 small invertebrates. Herbivorous pinfish adults, which 

 show a distinct dietary preference for Syringodium 

 over other seagrasses (Stoner 1980b), showed 

 largest relative abundance at the Syringodium site. 



Predation experiments also showed that predatory 

 efficiency is less affected by dense seagrass for small 

 juvenile pinfish than for large juvenile and subadult 

 size classes (Stoner 1982); therefore, the inverse 

 relationship between abundance of large pinfish and 

 seagrass biomass in Apalachee Bay may be a function 

 of decreasing predatory efficiency in heavy seagrass. 

 Conversely, small pinfish, adept at picking small prey 

 from among seagrass blades, show an abundance 

 pattern directly related to seagrass biomass and, 

 hence, food abundance. In correspondence with 

 these observations, it is interesting to note that nega- 

 tive correlations of fishes with seagrass biomass in 

 January, February, and March occurred with numeri- 

 cal dominance by early juvenile fishes which feed in 

 the water column or on small invertebrates of the 

 sediment such as harpacticoid copepods (Stoner 

 1980b; Livingston 1982). Positive correlations oc- 

 curred during months when collections were 

 dominated by benthic carnivores (May to October). 

 This observation provides more evidence for the 

 utility of the "ontogenetic trophic unit" concept pro- 

 posed by Livingston (1980) and Stoner (1980b). 



As a more parsimonious explanation of the ap- 

 parent relationship between fish abundance and 

 seagrass biomass or species, individual fishes may 

 simply prefer areas of high blade density, regardless 

 of seagrass species. The selective advantage of such a 

 habitat preference would be obvious if heavy losses 

 to predation or shortages of appropriate food types 

 occur outside the beds. Careful experimentation will 

 be required to determine which mechanisms are in- 

 volved in the choice of habitat by seagrass-associated 

 fishes. 



Heck and Orth (1980) have suggested that high 

 abundance and species richness of fishes and motile 

 invertebrates in seagrass meadows are, at least, par- 

 tially due to protection offered by seagrass blades. 

 Experimental evidence for this conclusion exists for 

 crustaceans (Nelson 1979; Coen et al. 1981; Stoner 

 1982), and it is likely that small fishes are rapidly 

 removed from unvegetated and sparsely vegetated 

 habitats by large, piscivorous predators. Large pis- 

 civores are abundant in the seagrass meadows of 

 Apalachee Bay (e.g., Cynoscion nebulosus) and are 



known to consume juvenile fishes such as Lagodon 

 rhomboides (Ryan 1981). The predatory efficiency of 

 most fishes would be reduced with increasing 

 seagrass biomass or blade density. Seine hauls con- 

 ducted by Gilmore 4 near the trawl stations in Indian 

 River also yielded large numbers of piscivorous 

 fishes which could have a significant effect on pop- 

 ulations of smaller fishes such as L. rhomboides, Eu- 

 cinostomus species, and Orthopristis chrysoptera. 

 Fifty-three percent of all seine hauls yielded snook, 

 Centropomus undecimalis, 41% contained great 

 barracuda, Sphyraena barracuda, and 627c contained 

 large grey snapper, Lutjanus griseus. Because these 

 piscivores found in the seagrass meadows are visual 

 predators relying on high speed, increasing blade 

 density probably hinders both prey detection and cap- 

 ture. The relatively low density of thin Syringodium 

 blades undoubtedly increases foraging efficiency of 

 large predators. Despite low biomass, high blade densi- 

 ty in Halodule beds (often over 10,000 blades/m : ) 

 may provide excellent protection for small and/or 

 juvenile fishes. High biomass and long, wide blades of 

 Thalassia may provide better protection for juveniles 

 than Syringodium, despite low blade densities. 



Seagrass biomass had very little effect on the 

 species richness of ichthyofauna in Apalachee Bay, 

 unlike the relationship shown for invertebrates 

 (Heck and Wetstone 1977; Stoner 1980a). Rather, 

 species richness and occurrence of certain species 

 appeared to be related to the presence of particular 

 microhabitats. Highest species richness was found at 

 the site with low plant biomass (station F-12), where 

 there is a patchy distribution of grasses with oc- 

 casional clumps of red algae and sponges. The impor- 

 tance of the red algal microhabitat has been reported 

 for fishes from both Apalachee Bay (Stoner and 

 Livingston 1980) and Indian River lagoon (Kulczycki 

 et al. 1981). Similarly, Weinstein and Heck (1979) 

 found that latitudinal variation in the richness of 

 seagrass-associated ichthyofauna was related to the 

 presence of non-seagrass habitats such as coral reefs 

 and mangroves. Consequently, increasing habitat 

 heterogeneity within beds may be more important 

 than seagrass biomass, species, or blade density in 

 determining species richness in fish communities. 



Although certain, highly mobile predators such as 

 sharks are known to move into seagrass meadows of 

 Apalachee Bay and the total number of fishes collect- 

 ed per trawl increases, the smaller resident species 

 appear not to leave the beds at night (Ryan 1981) as 

 some do in tropical regions where regular diurnal 



"R. G. Gilmore, Staff Scientist, Harbor Branch Foundation, Inc., 

 R.R. 1, Box 196, Fort Pierce, FL 33450, pers. commun. April 

 1981. 



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